Add buildpack_key to staging response

Send information about the admin buildpack used for staging back to the
cloud controller so it can generate app usage events that contain
buildpack information.
Merge branch 'add-bp-key-to-staging-response' of https://github.com/sykesm/stager into sykesm-add-bp-key-to-staging-response
Goderps

Signed-off-by: David Varvel <dvarvel@pivotallabs.com>